Mechanistic investigation of ammonium nitrogen adsorption on low-temperature pyrolysis cotton stalk biochar based on DFT calculations

The escalating nitrogen pollution from large-scale livestock wastewater and the urgent demand for agricultural waste valorization jointly drive the development of high-performance ammonium nitrogen (NH₄⁺-N) adsorbents. Herein, a series of cotton stalk biochars were fabricated through precisely controlled low-temperature pyrolysis (250–350 °C). Adsorption mechanisms were elucidated by integrating kinetic experiments with density functional theory (DFT) calculations. Key findings demonstrate that CS300II (pyrolyzed at 300 °C) achieves an adsorption capacity of 4.29 mg/g in simulated wastewater. Adsorption kinetics and isotherms exhibit excellent fitting to pseudo-first-order (R² > 0.987) and Langmuir (R² > 0.997) models, respectively, with a theoretical maximum capacity of 4.888 mg/g. Although this adsorption capacity is moderate compared to chemically modified or engineered adsorbents, the present study prioritizes mechanistic understanding, low energy input, and sustainability over maximum capacity. Although inhibited by coexisting ions in actual manure wastewater (1.81 mg/g), the adsorption mechanism remains consistent with simulated systems. Quantitative analysis of competing cations (K⁺, Ca²⁺, Na⁺) reveals that divalent cations exert the strongest inhibitory effect on NH₄⁺-N uptake. Multiscale characterization confirms adsorption is governed by oxygen-containing functional group coordination, synergistically assisted by electrostatic attraction, ion exchange, and hydrogen bonding. DFT calculations, performed on idealized carbon frameworks to capture qualitative mechanistic trends rather than absolute quantitative predictions, confirms that pyridine-N and carboxyl groups serve as the most energetically favorable adsorption sites. These computational findings are integrated with experimental evidence from FTIR, XPS, and zeta potential analyses to construct a unified multi-mechanism adsorption model. This work establishes mechanistic foundations for “pollution reduction–resource recovery” co-governance and guides the design of agricultural “carbon–nitrogen” composite materials.